This apparatus is an alternate approach to the structure shown in the above referenced patent which is drawn to a mud flow powered system which utilizes hydraulic oil, pumps and other apparatus more aptly shown in the patent to form output signals which encode the variables of interest. By contrast, this apparatus is directed to a hydraulic system utilizing an electrically powered solenoid valve output arrangement. To this end, the apparatus is triggered into operation by termination of drilling activities so that certain variables can be encoded and transmitted to the surface. As an example, three variables of interest in a downhole situation include heading, inclination and orientation. Inclination is the deviation of the tool at its downhole location compared with a vertical direction determined by a plumb bob. Heading refers to the direction of the hole in azimuth using north as a reference. Orientation refers to relative rotation of the tool with respect to a selected side of the tool. Inclination typically has a range of 0-90.0 degrees maximum, while heading and orientation both have a maximum range of 360.0 degrees.
These variables are measured and encoded. The encoding procedure utilizes an adjustable constriction in the tool to choke or constrain the back pressure on the mud flow path through the tool. This pressure variation can be sensed at the surface by detecting pressure variations in the mud flow line as mud is delivered to the drill string.
With the foregoing in mind, this apparatus is summarized briefly as a measuring while drilling tool which is installed in the drill string. It is compatible with a string of drill collars and is preferably located in the string of drill collars near the drill bit. To this end, it includes conventional pin and box connections and an axial passage therethrough for delivery of drilling mud. The apparatus utilizes a nose plug installed in the mud flow path which responds to pressure of the drilling mud. It is forced downwardly in response to pump pressure on the mud, and its downward movement charges a hydraulic system. The hydraulic system utilizes a three-position, four-way valve which is solenoid operated to control a constriction in the mud flow path. The constriction is made severe or is opened to the maximum capacity to thereby form pressure surges. The pressure surges have a variable duration to encode the signal of interest. The variable of interest is thus formed into an electrical signal for operation of a solenoid.